This invention relates to a process for the removal of sulfur compounds from waste gases. It particularly relates to a closed-cycle process for removing sulfur oxides from a power plant flue gas wherein a first stage of aqueous or solid absorption is followed by a two-stage regeneration process comprising the sequential steps of molten salt reduction and aqueous reformation. At the same time, hydrogen sulfide is formed and recovered as a source of sulfur values. In its preferred aspects, the sulfur dioxide content of a hot flue gas is removed by absorption in an aqueous sodium carbonate-bicarbonate solution or slurry using a spray-dryer scrubber to form a solid absorption product.
Sulfur oxides, principally as sulfur dioxide, are present in the waste gases discharged from many metal refining and chemical plants and in the flue gases from power plants generating electricity by the combustion of fossil fuels. The control of air pollution resulting from the discharge of sulfur oxides into the atmosphere has become increasingly urgent. An additional incentive for the removal of sulfur oxides from waste gases is the recovery of sulfur values otherwise lost by discharge to the atmosphere. However, particularly with respect to the flue gases from power plants, which based on the combustion of an average coal may contain as much as 3,000 p.p.m. sulfur dioxide and 30 p.p.m. sulfur trioxide by volume, the large volumes of these flue gases relative to the quantity of sulfur which they contain make removal or recovery of the sulfur compounds from these gases expensive. Also, while the possible by-products, such as elemental sulfur and sulfuric acid, that may be ultimately obtained from the recoverable sulfur values have virtually unlimited markets as basic raw materials, they sell for relatively low figures. Consequently, low-cost recovery processes are required.
Many processes have been proposed and investigated for the desulfurization of flue gases. Several dry processes have been proposed in which sulfur dioxide is removed either by chemical reaction with a solid absorbent or by adsorption on its surface followed by oxidation of the adsorbed sulfur dioxide. In one such process, shown in U.S. Pat. No. 2,718,453, finely powdered calcium carbonate is blown into the combustion gas to form calcium sulfate or calcium sulfite. In general, a reaction between a solid and gas is relatively slow and inefficient, being limited by the available surface area of the solid. Also, certain of the resultant products do not readily lend themselves to regeneration of the starting material or recovery of the removed sulfur values.
In the molten carbonate process shown in U.S. Pat. Nos. 3,438,722, 3,438,727, and 3,438,728, sulfur oxide impurities are removed from a hot combustion gas by contacting it at a temperature of at least 350.degree.C with a molten salt mixture containing alkali metal carbonates as the active absorbent. The spent absorbent is then regenerated chemically and recirculated. The adaptation of such a process to many older existing power-plant utility installations often presents certain economic disadvantages because of the requirements for modifying the boiler systems of these utility plants in order to obtain the flue gas to be treated at the required elevated temperature for the molten salt absorption rather than at its generally much lower exit temperature from the boiler.
Wet absorption processes are suitable for treating these lower temperature flue gases. In typical wet absorption processes, the flue gas is washed with an aqueous alkaline solution or slurry. Thus the use of an aqueous slurry of calcium hydroxide or calcium carbonate has been investigated in several British power plants. Also, aqueous sodium sulfite or ammonia solutions have been utilized as washing fluids.
In the wet absorption process shown in U.S. Pat. No. 3,533,748, a waste gas stream containing sulfur oxides is scrubbed with an aqueous solution of a soluble alkali such as sodium carbonate or sodium hydroxide to form sulfite and sulfate in solution. The resulting solution is then cooled to precipitate solid alkali metal sulfite and sulfate salts, which are separated from the solution and further processed.
While these wet absorption processes have some advantages, they all suffer from the common drawback of the flue gas being cooled substantially and becoming saturated with water vapor in the absorption tower. This cooling of the gas decreases the overall efficiency of the process because of the additional power requirements for dispersal of the flue gas to the atmosphere. Further, the associated condensation and precipitation of evaporated water containing contaminants in the surrounding environment, and the general formation of plumes at the point of emission from the power-plant stack, create substantial problems. Also, difficulties arise where economic and efficient recovery of the dissolved absorbent and sulfur values from aqueous solution is attempted. In many such processes, the recovery of elemental sulfur, a preferred product, is not economical.
In U.S. Pat. No. 3,305,307 is shown a process for the manufacture of solid alkali metal sulfite with negligible formation of alkali metal sulfate. A finely dispersed concentrated aqueous solution of an alkali metal compound such as sodium or potassium carbonate, hydroxide, or bicarbonate is passed into a substantially dry gas containing an equivalent or greater amount of sulfur dioxide, the dry gas being maintained at a temperature such that solid alkali metal sulfite is formed. To obtain the pure alkali metal sulfite by such a process, an excess reactant amount of SO.sub.2 compared with the alkali metal compound is required. Also, to avoid the formation of alkali metal sulfate, the gas containing the SO.sub.2 reactant must be relatively free of sulfur trioxide and oxidation-promoting substances such as nitrogen oxides and metal oxides, the latter being found as fly ash. In addition, a relatively low temperature of reaction is generally required, higher temperatures promoting formation of sulfate.
With respect to the proposed reduction and reformation steps herein, the reduction of sodium sulfate with carbon is generally known, particularly in connection with pulping operations. Illustrative are U.S. Pat. Nos. 1,130,317, 1,609,615, and 3,248,169. Gasification and combustion of carbonaceous materials are shown in U.S. Pat. Nos. 3,533,739 and 3,567,412. U.S. Pat. Nos. 2,344,104 and 2,838,374 also deal with the reduction of sulfites and sulfates. The aqueous reformation step has been generally shown in the art in connection with neutral sulfite semichemical pulping and Kraft processes. Illustrative are U.S. Pat. Nos. 2,163,554, 2,611,682 and 3,496,550.
The present process differs from known processes in providing an improved aqueous absorption process as well as a novel combination of the sequential steps of solid or aqueous absorption, molten salt reduction, and aqueous reformation to provide a closed-cycle process particularly advantageous for treating lower temperature flue gases containing sulfur oxides and for recovering the sulfur values present while regenerating the absorbent.